This invention relates to epoxy resin systems for use in prepreg applications. More particularly, this invention relates to aromatic diamines useful as hardeners for epoxy resins and to resin systems comprising epoxy resins and particular diamine hardeners. The resin systems are useful in producing prepreg and advanced composites.
Advanced composites are high strength, high modulus materials which are finding increasing use as structural components in aircraft, automotive, and sporting goods applications. Typically they comprise structural fibers such as carbon fibers in the form of woven cloth or continuous filaments embedded in a thermosetting resin matrix.
Most advanced composites are fabricated from prepreg, a ready-to-mold sheet of reinforcement impregnated with uncured or partially cured matrix resin. In order to be useful in commercial fabrication operations, prepreg matrix resin needs to have a long "out-time," defined as the period of time the prepreg can remain at room temperature and still be useful for making composites. Such prepreg is generally pliable and preferably has tack. Pliability is conferred by the matrix, which should remain soft and deformable without cracking in such applications. For epoxy-based prepreg systems comprising an epoxide resin and aromatic amine hardener, 4,4'-diaminodiphenylsulfone (DDS) has been the preferred hardener. DDS has a low level of reactivity with epoxy resins at room temperature and prepreg made using DDS-based systems has the desired long out-times.
Although cured composites prepared from DDS based matrix resins exhibit a useful combination of mechanical properties, the composites absorb moisture which detrimentally affects the high temperature properties and dimensional stability of fabricated parts. In addition, most DDS-cured formulations are brittle and the cured composites have poor impact resistance. Thus, there is a need for epoxy resin hardeners that exhibit the low room temperature reactivity of DDS while providing epoxy-based composites having reduced moisture sensitivity and improved toughness.